Fredrik Reinfeldlt also said that the then enterprise minister, Maud Olofsson, had asked the energy giant whether a purchase of the Dutch company would endanger Vattenfall's profit margins. The 2009 deal led to significant losses for Swedish taxpayers when Vattenfall wrote down its assets in 2013.
"The board chairman gave a reassuring response to the enterprise minister," he told a special inquiry held by the parliamentary committee on constitutional affairs (Konstitutionsutskottet - KU). "That was the background when the enterprise minister said that the owner had no objections to the purchase."
Ultimately, however, the board was responsible for the deal, Reinfeldt said. All board members voted in favour of the purchase.
Last year, after the asset write-down, Sveriges Television (SVT) reported that Vattenfall had been warned that such a deal may not add to its long-term value. Management consultants McKinsey had concluded that "a bigger buy would entail risk and would not create value per se".
Reinfeldt himself knew nothing of the deal until it was made public, he said on Thursday. He had known there were talks about Vattenfall's future going on over at the enterprise ministry, but said he had not been privy to any details.
As part of the KU inquiry, however, Social Democrat MP Hans Holm asked why Olofsson claimed she had told all the party leaders about Vattenfall's plan, a statement that seemingly contradicted the prime minster's pledge of ignorance on Thursday.
"I don't know where that comes from, but it doesn't reflect what happened here," Reinfeldt said.